In digital image processing, there is a distinct advantage to being able to electronically magnify, reduce or rotate the digital data coincident with the actual scanning of the document. Generally, the advantage is manifest in the ability to run the video capture and processing hardware at a predetermined video rate up to the image scaling circuitry and thereafter increasing or decreasing the video rate. Hence, the hardware may be designed for a specific operating speed, without having to continuously respond to changes in signal timing or frequency. A primary requirement in such operations is to provide interpolated pixel values and locations, that will accurately reflect the image in new data.
Two of the techniques which have been proposed for image scaling include nearest-neighbor and linear interpolation. Techniques of this type have applicability across a wide range of scanning products, for example the Xerox.RTM. 7650 Pro Imager.RTM. and electronic reprographic systems such as the Xerox.RTM. DocuTech Production Publisher.RTM.. The following disclosures may be relevant with regard to some of the approaches used for digital image scaling:
U.S. Pat. No. 4,275,450 to Potter discloses a magnification/demagnification apparatus and method wherein an accumulator is used to accumulate a reduction ratio, which, when greater than a predefined threshold, signals that the image signal should be passed on. Otherwise, the image signal is deleted and an additive reset count is added to the accumulator. In an alternative embodiment, an add-one circuit generates a signal which temporarily increases a magnification counter setting by one. The add-one circuit is only activated at selected times when the value in the magnification counter exceeds a threshold value.
U.S. Pat. No. 4,587,621 to DuVall discloses a device for magnification of image signals wherein a linear interpolator is used to calculate magnification ratios. The interpolator determines a spatial relationship of output values, with respect to a pair of digital scan values, where the spacing between successive scan outputs is defined as the reciprocal of a magnification value. A hardware circuit, which accumulates a magnification value and subsequently interpolates an input image to determine if an output pixel is valid, is used. Furthermore, the accumulated value is also used as an input to a large look-up table to determine the interpolated output value.
U.S. Pat. No. 4,742,553 to Irwin discloses a resolution conversion system for bitmap images using error term averaging, wherein an error value is added to an input value to generate an output value and a test value. An algorithm is disclosed which implements the resolution conversion and is particularly suitable for greyscale processing.
U.S. Pat. No. 4,809,083 to Nagano et al. discloses an image recording/reading apparatus with enlarging and reducing functions, wherein a desired magnification is divided into an integer part and a decimal part. The decimal part is then used to correct an image output. Every time the accumulated decimal part is greater than one, an output pixel is deleted.
U.S. Pat. No. 4,918,542 to Nagashima et al. discloses an image processing apparatus capable of smooth multi-area variable magnification, wherein an error term is determined by comparing a desired image magnification ratio with an actual image magnification ratio. Two counters are initially set to zero and count a number of output pixels and a number of input pixels. A signal, RDEB, stores an error term which is used to pad image output data. An algorithm is shown in FIGS. 10A, 10B, 11A, and 11B which pads an input image. The algorithm also determines if an output pixel is valid.
U.S. Pat. No. 5,008,752 to Van Nostrand discloses an interpolator for enlarging or reducing a digital image in two dimensions, which includes row and column interpolators. Both interpolators operate so as to generate signals indicating when the next element, row, or pixel is to be retrieved. The interpolators also produce a displacement address which is used to retrieve an interpolation coefficient from a look-up table, the interpolation coefficient being used subsequently to produce the interpolated output.
U.S. Pat. No. 5,025,405 to Swanson discloses a subsystem capable of image synthesis in a computer workstation. In general, the images to be displayed are represented by a function, wherein the values across an area, i.e., color or intensity, must be interpolated to accurately reflect the image. The system employs interpolation, based on precomputed integer and fractional portions of the function's slope, to scale the function using integer arithmetic
U.S. Pat. No. 5,237,432 to Calarco et al. shows interpolation in one dimension.
WO-91/01527 by Newman discloses an interpolation system that generates an interpolated value in response to an input value. The system includes a function value store, an address generator, and an interpolator. In operation, the address generator is used to generate address signals to identify locations in the function value store, which in turn provides function values that are used in conjunction with the input value to generate the interpolated value.
The cited references are incorporated by reference for their teachings.